1. Field of the Invention
The present invention relates to novel compositions based on ceric oxide, and, more especially, to novel compositions based on ceric oxide and comprising stabilizing amounts of silicon values, and which have high specific surfaces at high temperatures.
This invention also relates to a process for the preparation of such compositions and to the use of same, particularly in the field of catalysis.
As utilized herein, by the term "specific surface" is intended the B.E.T. specific surface area determined by nitrogen adsorption in conformity with ASTM standard D 3663-78 established from the BRUNAUER/EMMETT/TELLER technique described in the Journal of the American Chemical Society, volume 60, 309 (1938). The specific surfaces reported are measured on products which have been subjected to a calcination operation at the temperature and for the period of time indicated.
Moreover, the phrases "composition based on ceric oxide containing silicon," "ceric oxide stabilized by silicon," or, more simply, "stabilized ceric oxide" are used interchangeably herein, to specify the same final product.
2. Description of the Prior Art:
It is known to this art that ceric oxide may be used as a catalyst or catalyst support. Compare the research of Paul Meriaudeau et al, relating to the synthesis of methanol from CO+H.sub.2 over catalysts of platinum deposited on ceric oxide, reported at CR. Acad. Sc. Paris, t. 297, Series II--471 (1983).
It is also well known to this art that the efficiency of a catalyst is generally greater, the greater the contact surface between the catalyst and the reactants. To achieve this, it is necessary to maintain the catalyst in the most finely divided state possible, namely, the solid particles comprising same must be as small and as separate as possible. The fundamental role of the support is therefore to maintain the catalyst particles or crystallites in contact with the reactants, in the most finely divided state possible.
However, during prolonged use of a catalyst support, a decrease in its specific surface occurs, which may be caused either by the coalescence of the very fine micropores, or by growth of the crystallites thereof. During such coalescence, an amount of the catalyst becomes absorbed into the mass of the support and thus is no longer available for contact with the reactants.
This is the case with most of the ceric oxides to date known to this art, which have a specific surface which rapidly decreases at operational temperatures in excess of 500.degree. C.
Thus, R. Alvero et al, J. Chem. Soc., Dalton Trans, 87 (1984) have obtained from ceric ammonium nitrate a ceric oxide having, after calcination at a temperature of 600.degree. C., a specific surface of only 29 m.sup.2 /g.
FR-A-2,559,754 describes a ceric oxide having a specific surface of at least 85.+-.5 m.sup.2 /g, measured after calcination at a temperature ranging from 350.degree. to 450.degree. C. and, preferably, ranging from 100 to 130 m.sup.2 /g after calcination at a temperature of from 400.degree. to 450.degree. C. Said oxide is prepared by hydrolysis of an aqueous ceric nitrate solution in a nitric acid medium, followed by separation of the resulting precipitate, washing with an organic solvent, optional drying, and then calcination. The ceric oxide produced has a specific surface which is of value when it is used in the calcination temperature range of from 300.degree. to 600.degree. C. However, a decrease in the specific surface after calcination at a higher temperature is observed, this specific surface being 10 m.sup.2 /g, after calcination at 800.degree. C.
Also compare FR-A-2,559,755, which relates to a ceric oxide having a specific surface of at least 85.+-.5 m.sup.2 /g after calcination at a temperature of from 350.degree. to 500.degree. C., and preferably ranging from 150 to 180 m.sup.2 /g after calcination at a temperature of from 400.degree. to 450.degree. C. This oxide is obtained via a process which entails precipitating a basic ceric sulfate by reacting an aqueous ceric nitrate solution with an aqueous solution containing sulfate ions, separating the resulting precipitate, washing it with an ammonia solution, optionally drying it, and then calcining it at a temperature ranging from 300.degree. to 500.degree. C. The ceric oxide thus prepared has a high specific surface, but when it is subjected to a calcination operation at 800.degree. C., its specific surface considerably decreases, to a value on the order of 10 m.sup.2 /g.
In order to improve the stabilization of the specific surface of ceric oxides at high temperatures, EP-A-207,857, assigned to the assignee hereof, describes the utilization of various stabilization agents therefor, and more particularly silicon oxide. This additive (or dopant/doping agent) may be introduced either by impregnation of a presynthesized ceric oxide or during the manufacturing of the ceric oxide itself. In this latter case, a precursor of silicon oxide is introduced into a sol of a cerium IV compound, the resulting mixture is precipitated by addition of a base, and the precipitate recovered is then calcined.
Although the stabilized ceric oxides thus obtained have, as regards their specific surface, a substantially improved temperature behavior when compared with products devoid of the silicon addition, such behavior remains apparently insufficient as, for a 6-hour calcination at 800.degree. C., the best product obtained has a specific surface of only 36 m.sup.2 /g; moreover, if heated to 1,000.degree. C., the products experience a decrease in their specific surface to less than 10 m.sup.2 /g. Too, also per EP-A-207,857, it is indicated that, to attain acceptable stabilization, quantities on the order of 2.5% by weight of SiO.sub.2 must be used. However, in such amounts, the presence of silicon within the ceric oxide presents a problem for certain catalytic applications (poisoning by reason of the silicon).